1. Field of the Invention
The present invention relates generally to a system and method for cooling motors and more specifically to a system and method for cooling linear motors of a lithographic tool or device.
2. Background Description
An exposure apparatus utilizes one or more linear motors to precisely position a wafer stage which holds a semiconductor wafer relative to a reticle. The images transferred onto the wafer from the reticle are extremely small, and precise positioning of the wafer and the reticle is thus critical to the manufacturing of the wafer. To maintain precise control of the positioning of the wafer and reticle, a measuring system is utilized such as a laser interferometer in combination with a wafer stage mirror. The laser interferometer, which is known to be sensitive to air temperature variations, preferably has a wavelength of approximately 630 nm (i.e., red helium-neon laser).
Several types of linear motors are known to be used with exposure apparatus such as lithographic tools. In a typical linear motor, a magnet array encircles a coil array. The coil array includes a plurality of coils that are individually supplied with an electrical current. The electrical current generates an electromagnetic field that interacts with the magnetic field of the magnet array thus causing the coil array to move relative to the magnet array. When the magnet array is secured to the wafer stage, the wafer stage moves in concert with the magnet array. In this manner, precise movement of the wafer stage can be accomplished in several planes using several linear motors.
In providing an electrical current to the coils, however, heat is generated due to the resistance in the coils. The heat generation rate increases as the linear motor acceleration increases due to increased current requirements. The heat generated from the coils is subsequently transferred to the surrounding environment, including the air surrounding the linear motor and the other components positioned near the motor.
Several problems occur due to the heat generation in the linear motors. By way of a first example, the heat expands components of the lithographic machine, thus causing alignment problems and degrading the accuracy of the device. Additionally, the heat changes the index of refraction of the surrounding air which reduces the accuracy of the metrology system and degrades machine positioning accuracy. Specifically, the heat generated by the coils of the motor raises the air temperature around the motor which changes the index of refraction along the interferometer beam path. This causes noise and error in the interferometer signal. Moreover, the resistance of the coils increases as temperature increases which, in turn, exacerbates the heating problem and reduces the performance and life of the linear motor.
There is a requirement, then, to maintain the outer surface temperature of the linear motor housing at a very uniform temperature. That surface temperature must be matched closely to the surrounding air in the exposure apparatus. A typical specification is that the linear motor surface temperature be maintained within 0.1 C of the surrounding air temperature. Known systems have linear motors which operate at low power, and thus low speed, to minimize heat generation. However, whenever any current is supplied to the motor, heat is generated. That heat must be removed by conduction to the exposure apparatus frame and convection to the surrounding air. Heat removal by either conduction or convection requires that the motor surface be at higher temperature than the surroundings. Therefore, unless the motor is operated at extremely low power, its surface temperature will exceed the specification. Such a low-power motor cannot meet the other specifications for speed and acceleration.
In one solution, coolant such as the Novec series of coolants (manufactured by 3M) is used to cool the coils of the linear motor. The coolant flows through a close-fitting container around the coils carrying away the heat generated by the coils. There are two problems with this approach. First, as the industry builds faster stages, more coolant is required to absorb the heat generated by the coils. Secondly, the heat transferred to the coolant raises the coolant temperature. The temperature is higher on the outlet side of the coil container as compared to the inlet side. Such temperature non-uniformities cause the metrology problems discussed above. The physical properties of the coolant limit the amount of heat that can be absorbed for a given temperature rise. Specifically, the product of the coolant density and the coolant specific heat (the ρcp product) determines the coolant temperature rise for a given flowrate of coolant and heat dissipation in the coils. This ρcp product can be expressed in units of Watts/LPM−° K, where LPM is liter per minute. For example, one coolant may have a ρcp product of 30 Watts/LPM−° K. This means that for a coil power dissipation of 30 Watts and a coolant flow rate of 1 LPM, the coolant temperature rise will be 1° K. To decrease the temperature rise the coil power must be reduced or the coolant flow rate increased.
As can thus be seen, the ρcp product dictates the amount of coolant that must pass through the linear motor in order to remove the heat generated by the coils with an acceptable temperature rise. For example, as the coolant enters the coils at a controlled temperature, heat from the coils will raise the temperature of the coolant. As the coolant then leaves the linear motor, it will have a higher temperature. But, if the temperature rises too much, it will be necessary to pump more coolant through the linear motor to cool the coils of the motor. This requires a stronger pump, larger flow rates, additional energy needed for the system, large piping and additional flow-inducted vibration. If not enough coolant passes by the motor to carry away the heat, then the problems discussed above will result.